1. Field of the Invention
The present invention relates to an optical transmission system, and an optical transmitting device, an optical receiving device, and an optical transmission method used therein. More particularly, the present invention relates to a split-band optical transmission system, by which a frequency band is split, electrical signals are multiplexed, conversion to an optical signal is performed, and the resultant signal is transmitted, and an optical transmitting device, an optical receiving device, and an optical transmission method used therein.
2. Description of the Related Art
In recent years, the proliferation of broadband access increases the demand for a high-capacity transmission in a metropolitan area network and an access network. In response to such a demand, a high-speed optical transmission and a wavelength multiplexing optical transmission, which are mainly used in a backbone network, are being introduced into the metropolitan area network and the access network. The transmission speed of such a high-speed transmission exceeds 10 Gbps. However, compared to the backbone network, the metropolitan area network and the access network place a higher priority on cost-reduction, and require a configuration in which a high-speed transmission is realized at low cost.
When a high-speed optical transmission whose transmission speed exceeds 10 Gbps is performed, chromatic dispersion causes severe transmission waveform distortion. For example, in the case where an electrical signal is converted to an optical signal in the 1.55 μm range by using an optical transmitter, the optical signal is transmitted over a dispersion optical fiber, and the transmitted optical signal is converted to an electrical signal by using an optical receiver, the distortion of a transmission waveform becomes more pronounced. FIG. 19A is an illustration showing a frequency response of a conventional optical transmitter. FIG. 19B is an illustration showing a frequency response of a conventional optical transmission path composed of an optical transmitter, an optical receiver, and an optical fiber. FIG. 19C is an illustration showing a signal band used in conventional baseband transmission. In FIGS. 19A to 19C, a horizontal axis represents a frequency of an electrical signal input to the optical transmitter, and a vertical axis represents the level of the electrical signal. As shown in FIG. 19A, the optical transmitter has a frequency response whose band is wider than the signal band as shown in FIG. 19C. However, in the case where an optical signal in the 1.55 μm range is transmitted over the dispersion optical fiber, in general, it is impossible to transmit a signal in the vicinity of a frequency fm1 and a frequency fm2, as shown in FIG. 19B, due to chromatic dispersion. As such, as a transmission range becomes longer, a band reserved for an optical signal becomes narrower than the signal band. Thus, an optical signal can only be transmitted over a range in which a band of the optical transmission path is wider than a signal band. As a result, in the conventional configuration, it is difficult to perform a long-range transmission.
Thus, many methods enabling a long-range transmission by compensating chromatic dispersion have been developed. FIG. 20 is an illustration showing one example of an optical communication device utilizing a conventional chromatic dispersion compensation method disclosed in Japanese Patent Gazette No. 2760233. This method utilizes a dispersion compensation fiber whose chromatic dispersion is the inverse of chromatic dispersion of an optical fiber used in an optical transmission path. This optical communication device includes a transmitter 71, a dispersion compensation fibers 72 and 74, an optical transmission path 73, and a receiver 75. The dispersion compensation fibers 72 and 74 are set to have chromatic dispersion characteristics by which chromatic dispersion of the optical transmission path 73 is cancelled.
Other than the example as shown in FIG. 20, various types of dispersion compensation methods (e.g., a method in which a dispersion compensation fiber is replaced with a fiber grating) have been developed. These dispersion compensation methods are characterized in that an optical device whose chromatic dispersion is the inverse of chromatic dispersion of an optical transmission path is used for canceling chromatic dispersion of the optical transmission path.
However, in general, an optical device such as a dispersion compensation fiber used for chromatic dispersion is expensive, and it is difficult to apply such an optical device to the metropolitan area network and the access network.